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Sears RoadTalker 934.38260700 Dead... :-(

Justin B.

Justin B.

Active Member
Oct 15, 2024
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Hopefully somebody can help me with this or tell me it just needs to go into the scrap bin!

I picked this up a couple of years ago and stuffed it into the "someday" pile. The someday pile is nearing the end so I pulled it out the other day and started messing with it. Lights work, slight background hiss out of the speaker but no change when attaching antenna. Hooked signal to antenna input, set channel selector to multiple channels and adjusted sig gen output accordingly - no signal. Luckily I had a Sams for it (thanks Dan!) and started in on the alignments. The first step is to verify the 10.240 MHz signal at IC701, pin4 - no signal. I figured maybe a wonky crystal so I hooked my sig gen across it and injected a 10.240 MHz signal and the radio is still dead.

IC-701 is a SM5104 G PLL chip and maybe the chip is bad and and keeping the crystal from oscillating? I also spot-checked several inputs to the chip according to the truth-table and the levels were wrong and on wrong pins, etc.

Any suggestions on troubleshooting this thing? I've never had to deal with a PLL before and not really sure where to start!

TIA and 73,
 
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I have a few of those radios. One behaves like the one you have. I did start to recap the radio section by section, but still no luck. There is one part of the board I still need to recap, but I worked on the radio about 5+ years ago. Someday I will run into it again. :)
 
Not to put too fine a point on it, but did you verfy the PLL chip has power? Troubleshooting a dead radio should begin with an inventory of all the power supply branches and regulated voltages. A radio old enough to use zener diodes to regulate critical circuits especially. Dead zeners were a common failure back in the day when this was the common design choice.

73
 
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Thanks, Nomad. I can't remember but will check next time I get the cover off. Right now I'm in the middle of writing new Arduino code for my VFO. I may take a break from it later today and check.
 
My brain started getting tired so I pulled the RT40 back out. I probed around all over the place and none of the voltages made any sense. After beating my head on the bench for more than an hour I realized that I can't ground the meter to the chassis, like all other radios I've worked on, but chassis ground and "signal ground" are different. Kinda like the old Hallicrafters hot-chassis radios.

Tomorrow I will find a good ground reference to use with the meter, solder a wire to it, and continue on... :p

73,
 
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I have this exact radio with the exact issue, VCO is either floored at 5v or stalled at 0v. No 10.24, and pin 6 hangs at 5v too. I replaced C721, suspect for having 5v on the can body and swapped in a new 10.24. I isolated pin 6 thinking that the voltage may have been causing the issue with the internal oscillator, but its still pegged at 5v. I'm wondering if another PLL can be swapped in? Another SM5104 will be tough to find.
I'm sharing this just for notes to compare, if you get anywhere on the one you have.
 
Could you use a MC145104? I see one on the UK ebay.

The Defpom MC145104, SM5104, MM55104, MN6040A Component Info page

A page with component info for various cb radios
www.radiomods.co.nz

or make a daughter board that holds a more common and cheaper MC145106 and wire it to match?

Edit: other part numbers also cross reference to the same PLL. The NTE1255, the ECG1255, the MN6040A
 
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Thanks Brandon, Good plan.
I grabbed a Sylvania ECG1255 on eBay for $5, I'm going to give it a whirl. NEC1255 is another one I saw on there.

This is a cool looking radio, Id like to have it up and running. Otherwise the daughterboard idea was my original plan.

I'll post here again with the results.
I have plenty of old broken radios to monk around with. It's just finding the time that's an issue.
 
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I wonder if this is the same issue I am having on one of the 5 that I got. Just dead in the water and nothing. PLL is fine as I swapped it for another, and still wasnt working. I did swap out the daughterboard and to no avail. It behaved the same. Now I am wondering if the voltage regulators are toast. Please keep us updated on what your findings are. :)
 

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